This invention relates to an NOx concentration measuring apparatus utilizing two wide-range air/fuel ratio sensors.
For example, Japanese patent application No. 63-122707 describes an NOx concentration measuring apparatus which utilizes two wide-range air/fuel ratio sensors. The first sensor is sensitive to oxides of nitrogen for resolving oxides of nitrogen at low oxygen partial pressures. The first sensor includes an electrochemical cell containing an oxygen ion conductive electrolyte layer having two electrodes disposed on the opposite surfaces thereof for producing a first sensor signal indicative of an oxygen partial pressure of the gases introduced into the electrochemical cell. The second sensor is insensitive to oxides of nitrogen. The second sensor includes an electrochemical cell containing an oxygen ion conductive electrolyte layer having two electrodes disposed on the opposite surfaces thereof for producing a second sensor signal indicative of an oxygen partial pressure of the gases introduced into the electrochemical cell. The first and second sensor signals are used in calculating the NOx concentration of the gases introduced into the electrochemical cells of the first and second sensors.
The first and second sensors are located in the exhaust manifold of an internal combustion engine for measuring the NOx concentration of the exhaust gases discharged from the engine. However, the exhaust manifold pressure fluctuates greatly to affect the accuracy of the NOx concentration measurement of the first and second sensors during engine operation.